Plastics foams are synthetically produced materials with cell structure throughout their entire volume, with a density lower than that of the compact material from which they were produced. Their importance as lightweight construction material is constantly increasing, because they often have an unusual combination of mechanical properties with low densities. Thermal conductivity values are often very low, and therefore some foams have considerable importance as insulation material.
The disposal of plastics foams after use thereof is sometimes problematic. Another possibility often also available alongside energy recycling is materials recycling. However, biodegradability by way of example in a composting system would be desirable.
Polyester-based foams are known from various publications. These foams are usually produced by an extrusion process.
Said process has the disadvantage that it can generally produce only simple shapes such as blocks, sheets, and thin layers (foam foils). There is also often a restriction on the maximum thickness of blocks that can be produced by this process. In contrast, moldings of almost any desired geometry can be produced by using foam beads.
WO 2008/130226 describes by way of example the production of expandable polylactic acid beads, which are coated with special additives, and foam beads obtainable therefrom. In this case, extrusion processes are used. However, the mechanical properties of moldings produced therefrom are not always suitable for applications which require high tensile strengths and high compression strength.
The described problems can be avoided by using what is known as the autoclave process as described by way of example in Ullmann's Encyclopedia of Industrial Chemistry, vol. A11 1988. The range of requirements placed upon the foam beads thus produced and, respectively, the moldings resulting therefrom can differ greatly, depending on application sector. However, it is possible to define minimum requirements for the suitability of the process for various materials. These are:                The capability to produce foam beads over a wide range of bulk density, and in particular here it should be possible to achieve a very low bulk density irrespective of bead geometry and size. A low weight of the resultant moldings leads to cost savings.        The capability to produce foam beads with a very low particle size distribution (PSD) within a batch in a very large yield.        Foam beads with predominantly closed-cell foam structure. This leads to good processability in commercially available molding machines using the pressure-fill method, and to low water absorption.        Complete impregnation of the polymer beads to give foam beads without compact core. (No impairment of mechanical properties or of haptic properties of the resultant moldings.)        
Numerous parameters can be varied in the autoclave process in order to comply with these requirements. These are inter alia the suspension medium, the type of blowing agent and concentration thereof, the heating curve, and the impregnation temperature (IMT). It is not generally possible here to take a parameter, or parameter combination, that is particularly suitable for one material and use it for another material.
WO 00/68303 describes in general terms the production of expanded polymer beads based on biodegradable saturated polyesters in an autoclave process. Suspension medium preferably used here is ethylene glycol and glycerol with a density of from 1.1 to 1.25 g/cm3. These processes are not always entirely satisfactory in respect of the procedure, the capability of the process, and the property profile of the expanded foam beads thus obtainable.
EP 1683828 describes the production of expandable polymer beads on the basis of mixtures of different polylactic acid in an autoclave process. Very low impregnation temperatures and impregnation times of a number of hours are selected here. These expandable polymer beads are then foamed in a separate step to give expanded polymer beads.
EP 1378538 describes expanded beads based on mixtures of semicrystalline and amorphous polylactic acid, where the beads comprise at least 50% of polylactic acid. However, the autoclave production process usually takes place without liquid suspension medium with CO2 and impregnation temperatures below 30° C., and consequently impregnation times of a number of hours.
Operations in both applications do not take place in an aqueous suspension medium, and the overall description in both applications is of a process that is neither economic nor feasible.